1. Field of the Invention
The present invention relates to a semiconductor laser, particularly a high-output semiconductor laser for analog modulation.
2. Description of the Related Art
In recent years, subcarrier multiplexing (analog optical transmission) has been used in mobile communication systems, optical CATV systems, etc. As the light used in the subcarrier multiplexing, a semiconductor laser for analog modulation, having small intermodulation distortion is needed. This semiconductor laser is required to satisfy low distortion properties, i.e. a composite second order distortion (CSO) of -60 dBc or less and a composite triple beat (CTB) of -65 dBc or less in, for example, a 80-channel CATV system when the optical modulation index per channel is 3.5%. Further, the semiconductor laser is required to have a high output of 30 mW or more so that image distribution by one transmitter to as many viewers as possible can be achieved. Furthermore, the semiconductor laser is desired to be usable at a wide temperature range, for example, from -25.degree. C. to 65.degree. C.
As the light source for analog modulation, there have heretofore been widely used distributed feedback semiconductor lasers (DFB lasers) which can easily produce a single-mode oscillation. The DFB lasers, however, have had a problem in that they generate intermodulation distortion caused by the non-linearity of the current-light output property. They have generated large intermodulation distortion particularly at high temperatures and/or at high outputs because the non-linearity of the current-light output property is made conspicuous by the high-frequency leakage current, the leakage current flowing near the active layer and the light output saturation of laser caused by the insufficient breakdown voltage of the current-blocking layers.
To overcome the above problems, there is described a technique of forming an electrode mesa of narrow width for a lower device capacitance and thereby reducing the intermodulation distortion caused by leakage current, in, for example, FUJITSU SCIENTIFIC AND TECHNICAL JOURNAL, VOL. 29. NO. 4, DECEMBER, 1993. FIG. 15 is a conventional semiconductor laser. FIG. 15(a) shows a semiconductor laser of PBH (planar buried heterostructure) type. In FIG. 15(a), an active layer 1 is buried by a p-InP current-blocking layer 4, n-InP current-blocking layers 5 and a p-InP buried layer 6. In such a structure, the junction capacitance of the current-blocking layers cause leakage current; however, it is reported that the leakage current can be suppressed and reduction in intermodulation distortion is made possible by setting the width of the portion determined by an electrode mesa 11, i.e. the width (Wm) of the electrode mesa, at 7 .mu.m or less.
Conventional PBH semiconductor lasers, however, have had a problem in that they generate large intermodulation distortion at high temperatures and/or at high outputs. The first reason therefor is that in conventional semiconductor lasers of PBH type, the current-blocking layers have no sufficient voltage breakdown property at high temperatures and/or at high outputs, which makes conspicuous the light output saturation when the current is increased at high temperatures and/or at high outputs. The second reason is that since the high-precision control of the distance between each n-InP current-blocking layer and the active layer 1 is difficult in the process for production of conventional PBH semiconductor laser, the leakage current flowing near the active layer cannot be made sufficiently small, which makes conspicuous the light output saturation when the current is increased at high temperatures and/or at high outputs. Therefore, conventional PBH semiconductor lasers have found no application in, for example, a CATV system whose output exceeds 30 mW; moreover, they have had no sufficient stability in intermodulation distortion at a wide temperature range.
Meanwhile, a DC-PBH (double channel planar buried heterostructure) laser was proposed in, for example, ELECTRONICS LETTERS, VOL. 18, NO. 22, 1982. It is a semiconductor laser as shown in FIG. 15(b), having an active layer 1, one pair of channels 3 provided at the two ends of the active layer 1, and two recombination layers 2 of the same layer structure as the active layer 1, provided outside the channels 3; and is relatively superior in properties at high temperatures and/or at high outputs. This semiconductor laser has a further advantage in that since the distance between the active layer 1 and each n-InP current-blocking layer 5 can be controlled at a high precision, the leakage current flowing near the active layer can be made sufficiently small.
Even in this DC-PBH laser having the above-mentioned properties, however, the intermodulation distortion at high temperatures and/or at high outputs has been insufficient. The reason is that the electrostatic capacitance by the recombination layers 2 is large and it generates a high-frequency leakage current flowing through the recombination layers 2.